1. Field of the Invention
This invention relates to a process for the recovery of precious metals such as gold and silver from carbonaceous ores and mixtures of carbonaceous and oxide ores containing such metals by leaching and cementation techniques.
2. Prior Art
The present practices in the field of recovery of precious metals such as gold and silver from ores often require segregation of such ores prior to their processing, of which ores there are two basic types: first, oxide ores from which the precious metal values are easily extracted by present cyanidation techniques, and, second, carbonaceous ores which are refractory to conventional cyanidation techniques and which are characterized by their organic carbon content, which is normally between 0.25 and 3% by weight. To render the latter more amenable to cyanide extraction a single-or multi-stage pretreatment prior to cyanidation is normally required to prevent the carbonaceous component of the ore from adsorbing the gold- or silver- cyanide complex formed during leaching. This pretreatment alone can consume up to approximately thirty hours of processing time and necessitates costly plant equipment and operating expenditures.
Various patents have separately addressed the leaching and cementing aspects of processing ores bearing precious metals to effect improved recoveries. U.S. Pat. No. 805,017, for example, discloses the synergistic effect of leaching ores with solutions of sodium cyanide and ammonium carbonate, along with other ammonium additives such as ammonium nitrate, and compressed air. The chemistry which this particular system of leaching affords reportedly involves complete dissolution of the precious metal values present in the ore via complexation with the cyanogen formed in solution. Carbon dioxide produced in such system, resulting from the addition of the ammonium carbonate, protects the cyanogen from destruction by ammonia, and the ammonium ions present in solution are said to provide better extractions. After the extraction process is performed phase disengagement is carried out to obtain a metal-bearing solution from which the precious metal values may be recovered by any suitable means.
A process for the extraction of gold and silver from their ores employing an alkaline cyanide compound as the leaching agent wherein the precious metal values are extracted from the ore followed by filtration of the leaching slurry to obtain a solution which will afford recovery of the dissolved metal values via cementation is taught in U.S. Pat. No. 4,177,068. The concept of simultaneous leaching and cementation is not disclosed in this patent.
U.S. Pat. Nos. 512,046 and 513,174 discloses methods wherein cyanide solutions containing precious metal values can be subjected to cementation with aluminum and zinc, respectively. Both patents require the use of filtered solutions for cementation and fail to teach a simultaneous leaching and cementation process. The same may be said of U.S. Pat. No. 418,137 which claims cyanide leaching of neutralized ores to cause the dissolution of the metal values present in the ore followed by solid-liquid separation and passage of the metal-bearing solution obtained through a zinc sponge for cementation.
U.S. Pat. No. 1,002,446 describes a method of recovering precious metals from their ores wherein a finely divided ore is suspended in a cyanide solution, which solution has effected the dissolution of the metal values present in the ore, with the introduction of aluminum plates directly into the ore pulp to afford cementation. This patent calls for completeness in dissolution of the precious metals contained in the ore.